Demoman

Demoman is a surprisingly versatile class that allows for fulfilling of multiple roles, but the primary role that he is assigned to is killing giants and groups of uber Medics. By using a sticky trap set by the Scottish Resistance, Demo can easily wipe out any group of Medics, and with a condensed pile of crit stickies he can easily one-shot kill every single standard giant in the game, making him highly favored when the team can hold the line close to the robots' spawn point. Stickies can also be used to wipe out crowds, but it's not the most consistent method of killing tiny robots and you'll more than likely need someone else to clean up the trash for you.

Outside of being the team's dedicated Medic and giant killer, a Demo can instead specialize in doing both single-target and AoE DPS by investing in his primary weapon, which can put out ridiculous amounts of burst damage when upgraded, with the downside of having to reload for a while after said burst. However, even if one focuses on using grenades, sticky traps should still be set for groups of uber Medics.

Others might prefer playing Demoknight instead, and interestingly enough you can get upgrades for your melee if you equip a shield in your secondary slot, meaning that the subclass is officially recognized in MvM. However, there are really only two things Demoknight is really good at: Busting the tank and chaining kills on tiny robots, preferably Demoknight bots. Demoknight is typically considered a more expensive and less efficient Pyro, and is discouraged for new players.

Common mistakes

As explained one paragraph ago, Demoknight isn't nearly as generally applicable as a Sticky-based or Grenade-based Demo. Yes, it is true that Demoknight can work with specific builds and on specific waves, but he's not the kind of class you should be running full-time. On early waves, you won't have enough resistances or survivability to be able to charge into a crowd, and even on later waves when you can afford Demoknight's expensive upgrades, the rest of your team will be powerful enough to wipe out all the tiny robots before you can run to them. In most circumstances, choosing to play Demoknight can be amounted to throwing or goofing off.

Both your primary and secondary weapon can be invested in to wreak havoc onto the robots, but it must be noted that Demoman is an expensive class to play, especially if you're investing into your primary. For this reason, when picking Demo you should decide on which of your two weapons you want to focus on, so that you can have one really good and fully upgraded weapon instead of two half-upgraded weapons.

Upgrade priority

Regardless of whether you choose to upgrade your primary or secondary, the upgrade path is generally the same. Max out reload speed as soon as possible, then full firing speed, then balance out getting damage and clip size. For both weapons, two ticks of clip size tend to be enough, though with grenades you might prefer getting 3 or 4 on the later waves. For stickies, two ticks of damage will be enough to one-shot kill every giant with a crit sticky pile from a Scottish Resistance, but it's a good choice to get 3 or 4 ticks of damage simply to make it more consistent at killing giants and tiny robots alike. On grenades, meanwhile, damage is sort of fluid - on the early waves, remember that two direct pipes are enough to kill every tiny robot short of a Heavy. However, splash damage is incredibly important and damage should be upgraded to make your grenades more reliably wipe out condensed groups of robots surrounding the ones who take your direct hits. If you plan on taking on the tank or giants with your primary, you should invest both into damage and clip size as much as possible. One last note: Projectile speed is an incredibly good value, and just by taking one or two ticks of the very cheap upgrade, you can hit everything more reliably and at a longer range.

Crit canteens should be used relatively sparingly, especially if you have a Medic reliably feeding you crits. If you do have to use crit canteens, only buy enough to kill the most important giants in a wave, such as giant Heavies, Soldiers, or Medics.

Because Demo's weapons are so expensive to upgrade, he often cannot afford many resistances. Indeed, you shouldn't really put credits into any resistances until the second or third wave, once you've fully upgraded reload speed and firing speed, and even then you shouldn't spend so much that you can't afford getting important upgrades for your weapon of choice.

Notes for sticky traps

You should only spread out your stickies in a wide carpet if you plan on killing a large amount of small robots all at once, this includes uber Medics. In all other scenarios, it is best to keep your stickies stacked onto one spot, where you can detonate them one by one by continually firing out more stickies, or to do huge damage against giants by detonating them all at once. If your stickies are critical, then you usually don't need to spread them out to kill uber Medics connected to a giant, unless there's like 15 of them at once.

When using a Scottish Resistance with upgraded clip size and firing speed, you can fire, at most, 11 stickies during a single crit canteen. A Kritzkrieg charge with one tick of uber duration, or an upgraded crit canteen from a Medic, will be enough to shoot 14 stickies, the maximum amount.

Here are some damage thresholds for killing giants:

Weapons

Primary

Secondary

Melee